Flowing vapor pressure apparatus and related method

ABSTRACT

A method of determining vapor pressure of a fluid is provided. The method includes the steps of providing a meter ( 5 ) having meter electronics ( 20 ), the meter ( 5 ) being at least one of a flowmeter and a densitometer, and flowing a process fluid through the meter ( 5 ). A pressure of the process fluid is measured. The pressure of the process fluid is adjusted until a monophasic/biphasic boundary is reached. The flowing vapor pressure of the process fluid is determined at the monophasic/biphasic boundary.

TECHNICAL FIELD

The present invention relates to vibratory meters, and moreparticularly, to a method and apparatus for real-time vapor pressuredetermination.

BACKGROUND OF THE INVENTION

Reid Vapor Pressure (RVP) is one of the most widely recognizedproperties for measuring and enforcing fuel quality standards. Flowingvapor pressure is an important property in applications which handleflow and storage of volatile fluids such as gasoline, natural gasliquids, and liquid petroleum gas. Vapor pressure provides an indicationof how volatile fluids may perform during handling, and furtherindicates conditions under which bubbles will likely form and pressurewill likely build. As such, vapor pressure measurement of volatilefluids increases safety and prevents damage to transport vessels andinfrastructure.

If the vapor pressure of a fluid is too high, cavitation during pumpingand transfer operations may occur. Furthermore, vessel or process linevapor pressure may potentially rise beyond safe levels due totemperature changes. It is therefore often required that RVP be knownprior to storage and transport. Typically, RVP is determined bycapturing samples and removing them to a laboratory for testing todetermine the value from the sample. This poses difficult issues forregulatory fuel quality standards enforcement because of the delay inobtaining final results, the cost of maintaining a lab, and the safetyand legal evidence vulnerabilities associated with sample handling.Flowing vapor pressure is often determined by this same process,followed by a conversion from the RVP determined in a lab to the flowingvapor pressure at flowing temperature by relying on lookup tables anddatabases based on empirical measurements.

A need therefore exists for an in-line device or system that can measureflowing vapor pressure and/or RVP on a continuous, real-time, basisunder process conditions. This is provided by the present embodiments,and an advance in the art is achieved. On-site measurement is morereliable, as it obviates the need for the periodic sampling and fullyeliminates the risk of fluid property changes between the time of samplecollection and laboratory assay. Furthermore, safety is improved byhaving real-time measurements, as unsafe conditions may be remediedimmediately. Additionally, money is saved, as regulatory enforcement maybe conducted via simple on-site checks, wherein inspection andenforcement decisions may be made with little delay or processcessation.

SUMMARY OF THE INVENTION

According to an embodiment, a method of determining vapor pressure of afluid is provided. The method comprises providing a meter having meterelectronics, wherein the meter comprises at least one of a flowmeter anda densitometer. A process fluid is flowed through the meter, and apressure of the process fluid is measured. The pressure of the processfluid is adjusted until a monophasic/biphasic boundary is reached, andthe flowing vapor pressure of the process fluid at themonophasic/biphasic boundary is determined. According to an embodiment,a system for determining flowing vapor pressure of a process fluid isprovided. The system comprises a meter comprising at least one of aflowmeter and a densitometer. A pressure regulator is in fluidcommunication with the meter. The system comprises a pressure sensor.Meter electronics is in communication with the meter and the pressuresensor, wherein the meter electronics is configured to receive ameasured pressure. The meter electronics is configured to control thepressure regulator to adjust the pressure of the process fluid until amonophasic/biphasic boundary is reached, and determine the flowing vaporpressure of the process fluid at the monophasic/biphasic boundary.

Aspects

According to an aspect, a method of determining vapor pressure of afluid, comprising the steps of: providing a meter having meterelectronics, wherein the meter comprises at least one of a flowmeter anda densitometer, flowing a process fluid through the meter, measuring apressure of the process fluid, adjusting the pressure of the processfluid until a monophasic/biphasic boundary is reached, and determiningthe flowing vapor pressure of the process fluid at themonophasic/biphasic boundary.

Preferably, the step of adjusting the pressure of the process fluiduntil the monophasic/biphasic boundary is reached comprises lowering apressure of a valve positioned upstream of the meter.

Preferably, the step of adjusting the pressure of the process fluiduntil the monophasic/biphasic boundary is reached comprises raising apressure of a valve positioned downstream of the meter.

Preferably, the method comprises measuring the temperature of theprocess fluid, and calculating the Reid Vapor Pressure from thetemperature and the flowing vapor pressure.

Preferably, the step of calculating the Reid Vapor Pressure from thetemperature and the flowing vapor pressure comprises referencing ReidVapor Pressure values stored in the meter electronics using the ReidVapor Pressure from the temperature.

Preferably, the Reid Vapor Pressure values stored in the meterelectronics comprise a look-up table.

Preferably, the Reid Vapor Pressure values stored in the meterelectronics are calculated from a curve.

Preferably, the method comprises determining the presence of entrainedgas in the process fluid with a measured drive gain.

Preferably, the method comprises determining the presence of entrainedgas in the process fluid by measuring a density of the fluid.

Preferably, the method comprises determining the presence of entrainedgas in the process fluid with a combination of a measured drive gain anda measured density.

According to an aspect, a system for determining flowing vapor pressureof a process fluid, comprises a meter comprising at least one of aflowmeter and a densitometer. A pressure regulator is in fluidcommunication with the meter. The system comprises a pressure sensor.Meter electronics are in communication with the meter and the pressuresensor, wherein the meter electronics is configured to: receive ameasured pressure, control the pressure regulator to adjust the pressureof the process fluid until a monophasic/biphasic boundary is reached,and determine the flowing vapor pressure of the process fluid at themonophasic/biphasic boundary.

Preferably, the system comprises: one or more conduits, and at least onedriver attached to the one or more conduits configured to generate avibratory signal to the one or more conduits. At least one pickoff isattached to the one or more conduits configured to receive a vibratorysignal from the one or more conduits.

Preferably, the system comprises a temperature sensor configured tomeasure the temperature of the process fluid, wherein the meterelectronics is configured to calculate the Reid Vapor Pressure from themeasured temperature of the process fluid and the flowing vaporpressure.

Preferably, the meter electronics comprises Reid Vapor Pressurereference values stored therein.

Preferably, the Reid Vapor Pressure reference values stored in the meterelectronics comprise a look-up table.

Preferably, the Reid Vapor Pressure reference values stored in the meterelectronics are calculated therein.

Preferably, the meter electronics is configured to determine thepresence of entrained gas in the process fluid with a measured drivegain.

Preferably, the meter electronics is configured to determine thepresence of entrained gas in the process fluid with a measured density.

Preferably, the meter electronics is configured to determine thepresence of entrained gas in the process fluid with a combination of ameasured drive gain and a measured density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a flowmeter sensor assembly according to anembodiment;

FIG. 2 illustrates meter electronics according to an embodiment;

FIG. 3 illustrates a vapor pressure determination system according to anembodiment; and

FIG. 4 illustrates a method of vapor pressure determination according toan embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-4 and the following description depict specific examples toteach those skilled in the art how to make and use the best mode of theinvention. For the purpose of teaching inventive principles, someconventional aspects have been simplified or omitted. Those skilled inthe art will appreciate variations from these examples that fall withinthe scope of the invention. Those skilled in the art will appreciatethat the features described below can be combined in various ways toform multiple variations of the invention. As a result, the invention isnot limited to the specific examples described below, but only by theclaims and their equivalents.

Vibrating sensors, such as for example, vibrating densitometers andCoriolis flowmeters are generally known, and are used to measure massflow and other information related to materials flowing through aconduit in the flowmeter or a conduit containing the densitometer.Exemplary flowmeters are disclosed in U.S. Pat. No. 4,109,524,4,491,025, and Re. 31,450, all to J. E. Smith et al. These flowmetershave one or more conduits of a straight or curved configuration. Eachconduit configuration in a Coriolis mass flowmeter, for example, has aset of natural vibration modes, which may be of simple bending,torsional, or coupled type. Each conduit can be driven to oscillate at apreferred mode. Some types of mass flowmeters, especially Coriolisflowmeters, are capable of being operated in a manner that performs adirect measurement of density to provide volumetric information throughthe quotient of mass over density. See, e.g., U.S. Pat. No. 4,872,351 toRuesch for a net oil computer that uses a Coriolis flowmeter to measurethe density of an unknown multiphase fluid. U.S. Pat. No. 5,687,100 toButtler et al., teaches a Coriolis effect densitometer that corrects thedensity readings for mass flow rate effects in a mass flowmeteroperating as a vibrating tube densitometer.

Material flows into the flowmeter from a connected pipeline on the inletside of the flowmeter, is directed through the conduit(s), and exits theflowmeter through the outlet side of the flowmeter. The naturalvibration modes of the vibrating system are defined in part by thecombined mass of the conduits and the material flowing within theconduits.

When there is no flow through the flowmeter, a driving force applied tothe conduit(s) causes all points along the conduit(s) to oscillate withidentical phase or with a small “zero offset”, which is a time delaymeasured at zero flow. As material begins to flow through the flowmeter,Coriolis forces cause each point along the conduit(s) to have adifferent phase. For example, the phase at the inlet end of theflowmeter lags the phase at the centralized driver position, while thephase at the outlet leads the phase at the centralized driver position.Pickoffs on the conduit(s) produce sinusoidal signals representative ofthe motion of the conduit(s). Signals output from the pickoffs areprocessed to determine the time delay between the pickoffs. The timedelay between the two or more pickoffs is proportional to the mass flowrate of material flowing through the conduit(s).

Meter electronics connected to the driver generate a drive signal tooperate the driver and also to determine a mass flow rate and/or otherproperties of a process material from signals received from thepickoffs. The driver may comprise one of many well-known arrangements;however, a magnet and an opposing drive coil have received great successin the flowmeter industry. An alternating current is passed to the drivecoil for vibrating the conduit(s) at a desired conduit amplitude andfrequency. It is also known in the art to provide the pickoffs as amagnet and coil arrangement very similar to the driver arrangement.However, while the driver receives a current which induces a motion, thepickoffs can use the motion provided by the driver to induce a voltage.The magnitude of the time delay measured by the pickoffs is very small;often measured in nanoseconds. Therefore, it is necessary to have thetransducer output be very accurate.

FIG. 1 illustrates a flowmeter 5, which can be any vibrating meter, suchas a Coriolis flowmeter or densitometer, for example without limitation.The flowmeter 5 comprises a sensor assembly 10 and meter electronics 20.The sensor assembly 10 responds to mass flow rate and density of aprocess material. Meter electronics 20 are connected to the sensorassembly 10 via leads 100 to provide density, mass flow rate, andtemperature information over path 26, as well as other information. Thesensor assembly 10 includes flanges 101 and 101′, a pair of manifolds102 and 102′, a pair of parallel conduits 103 (first conduit) and 103′(second conduit), a driver 104, a temperature sensor 106 such as aresistive temperature detector (RTD), and a pair of pickoffs 105 and105′, such as magnet/coil pickoffs, strain gages, optical sensors, orany other pickoff known in the art. The conduits 103 and 103′ have inletlegs 107 and 107′ and outlet legs 108 and 108′, respectively. Conduits103 and 103′ bend in at least one symmetrical location along theirlength and are essentially parallel throughout their length. Eachconduit 103, 103′, oscillates about axes W and W′, respectively.

The legs 107, 107′, 108, 108′ of conduits 103,103′ are fixedly attachedto conduit mounting blocks 109 and 109′ and these blocks, in turn, arefixedly attached to manifolds 102 and 102′. This provides a continuousclosed material path through the sensor assembly 10.

When flanges 101 and 101′ are connected to a process line (not shown)that carries the process material that is being measured, materialenters a first end 110 of the flowmeter 5 through a first orifice (notvisible in the view of FIG. 1) in flange 101, and is conducted throughthe manifold 102 to conduit mounting block 109. Within the manifold 102,the material is divided and routed through conduits 103 and 103′. Uponexiting conduits 103 and 103′, the process material is recombined in asingle stream within manifold 102′ and is thereafter routed to exit asecond end 112 connected by flange 101′ to the process line (not shown).

Conduits 103 and 103′ are selected and appropriately mounted to theconduit mounting blocks 109 and 109′ so as to have substantially thesame mass distribution, moments of inertia, and Young's modulus aboutbending axes W-W and W′-W′, respectively. Inasmuch as the Young'smodulus of the conduits 103, 103′ changes with temperature, and thischange affects the calculation of flow and density, a temperature sensor106 is mounted to at least one conduit 103, 103′ to continuously measurethe temperature of the conduit. The temperature of the conduit, andhence the voltage appearing across the temperature sensor 106 for agiven current passing therethrough, is governed primarily by thetemperature of the material passing through the conduit. Thetemperature-dependent voltage appearing across the temperature sensor106 is used in a well-known method by meter electronics 20 to compensatefor the change in elastic modulus of conduits 103, 103′ due to anychanges in conduit 103, 103′ temperature. The temperature sensor 106 isconnected to meter electronics 20.

Both conduits 103, 103′ are driven by driver 104 in opposite directionsabout their respective bending axes W and W′ at what is termed the firstout-of-phase bending mode of the flowmeter. This driver 104 may compriseany one of many well-known arrangements, such as a magnet mounted toconduit 103′ and an opposing coil mounted to conduit 103, through whichan alternating current is passed for vibrating both conduits. A suitabledrive signal is applied by meter electronics 20, via lead 113, to thedriver 104. It should be appreciated that while the discussion isdirected towards two conduits 103, 103′, in other embodiments, only asingle conduit may be provided or more than two conduits may beprovided. It is also within the scope of the present invention toproduce multiple drive signals for multiple drivers and for thedriver(s) to drive the conduits in modes other than the firstout-of-phase bending mode.

Meter electronics 20 receive the temperature signal on lead 114, and theleft and right velocity signals appearing on leads 115 and 115′,respectively. Meter electronics 20 produce the drive signal appearing onlead 113 to driver 104 and vibrate conduits 103, 103′. Meter electronics20 process the left and right velocity signals and the temperaturesignal to compute the mass flow rate and the density of the materialpassing through the sensor assembly 10. This information, along withother information, is applied by meter electronics 20 over path 26 toutilization means. An explanation of the circuitry of the meterelectronics 20 is not needed to understand the present invention and isomitted for brevity of this description. It should be appreciated thatthe description of FIG. 1 is provided merely as an example of theoperation of one possible vibrating meter and is not intended to limitthe teaching of the present invention.

A Coriolis flowmeter structure is described although it will be apparentto those skilled in the art that the present invention could bepracticed on a vibrating tube or fork densitometer without theadditional measurement capability provided by a Coriolis mass flowmeter.

FIG. 2 is a block diagram of the meter electronics 20 of flowmeter 5according to an embodiment. In operation, the flowmeter 5 providesvarious measurement values that may be outputted including one or moreof a measured or averaged value of mass flow rate, volume flow rate,individual flow component mass and volume flow rates, and total flowrate, including, for example, both volume and mass flow of individualflow components.

The flowmeter 5 generates a vibrational response. The vibrationalresponse is received and processed by the meter electronics 20 togenerate one or more fluid measurement values. The values can bemonitored, recorded, saved, totaled, and/or output.

The meter electronics 20 includes an interface 201, a processing system203 in communication with the interface 201, and a storage system 204 incommunication with the processing system 203. Although these componentsare shown as distinct blocks, it should be understood that the meterelectronics 20 can be comprised of various combinations of integratedand/or discrete components.

The interface 201 is configured to communicate with the sensor assembly10 of the flowmeter 5. The interface 201 may be configured to couple tothe leads 100 (see FIG. 1) and exchange signals with the driver 104,pickoff sensors 105 and 105′, and temperature sensors 106, for example.The interface 201 may be further configured to communicate over thecommunication path 26, such as to external devices.

The processing system 203 can comprise any manner of processing system.The processing system 203 is configured to retrieve and execute storedroutines in order to operate the flowmeter 5. The storage system 204 canstore routines including a flowmeter routine 205, a valve controlroutine 211, a drive gain routine 213, and a vapor pressure routine 215.The storage system 204 can store measurements, received values, workingvalues, and other information. In some embodiments, the storage systemstores a mass flow (m) 221, a density (ρ) 225, a density threshold(226), a viscosity (μ) 223, a temperature (T) 224, a pressure 209, adrive gain 306, a drive gain threshold 302, a gas entrainment threshold244, a gas entrainment fraction 248, and any other variables known inthe art. The routines 205, 211, 213, 215 may comprise any signal notedand those other variables known in the art. Other measurement/processingroutines are contemplated and are within the scope of the descriptionand claims.

The flowmeter routine 205 can produce and store fluid quantificationsand flow measurements. These values can comprise substantiallyinstantaneous measurement values or can comprise totalized oraccumulated values. For example, the flowmeter routine 205 can generatemass flow measurements and store them in the mass flow 221 storage ofthe storage system 204, for example. The flowmeter routine 205 cangenerate density 225 measurements and store them in the density 225storage, for example. The mass flow 221 and density 225 values aredetermined from the vibrational response, as previously discussed and asknown in the art. The mass flow and other measurements can comprise asubstantially instantaneous value, can comprise a sample, can comprisean averaged value over a time interval, or can comprise an accumulatedvalue over a time interval. The time interval may be chosen tocorrespond to a block of time during which certain fluid conditions aredetected, for example a liquid-only fluid state, or alternatively, afluid state including liquids and entrained gas. In addition, other massand volume flow and related quantifications are contemplated and arewithin the scope of the description and claims.

As noted, drive gain 306 may be utilized as the signal that indicates ano-flow/false totalizing condition. A drive gain threshold 302 may beused to distinguish between periods of flow, no flow, amonophasic/biphasic boundary, and gas entrainment/mixed-phase flow.Similarly, a density threshold 226 applied to the density reading 225may also be used, separately or together with the drive gain, todistinguish gas entrainment/mixed-phase flow. Drive gain 306 may beutilized as a metric for the sensitivity of the flowmeter's 5 conduitvibration to the presence of fluids of disparate densities, such asliquid and gas phases, for example, without limitation. The combinedeffect of damping on energy input and resulting amplitude is known asextended drive gain, which represents an estimate of how much powerwould be required to maintain target vibration amplitude, if more than100% power were available:

$\begin{matrix}{{{Extended}\mspace{14mu} {Drive}\mspace{14mu} {Gain}} = {{Drive}\mspace{14mu} {Gain}*\frac{{Drive}\mspace{14mu} {Target}}{\left( \frac{{Max}\left( {{{Left}\mspace{14mu} {Pickoff}},{{Right}\mspace{14mu} {Pickoff}}} \right)}{Frequency} \right)}}} & (1)\end{matrix}$

It should be noted that, for purposes of the embodiments providedherein, that the term drive gain may, in some embodiments, refer todrive current, pickoff voltage, or any signal measured or derived thatindicates the amount of power needed to drive the flow conduits 103,103′ at a particular amplitude. In related embodiments, the term drivegain may be expanded to encompass any metric utilized to detectmulti-phase flow, such as noise levels, standard deviation of signals,damping-related measurements, and any other means known in the art todetect mixed-phase flow. In an embodiment, these metrics may be comparedacross the pick-off sensors 105 and 105′ to detect a mixed-phase flow.

The vibrating conduits 103, 103′ take very little energy to keepvibrating at their first resonant frequency, so long as all of the fluidin the tube is homogenous with regard to density. In the case of thefluid consisting of two (or more) immiscible components of differentdensities, the vibration of the tube will cause displacement ofdifferent magnitudes of each of the components. This difference indisplacement is known as decoupling, and the magnitude of thisdecoupling has been shown to be dependent on the ratio of the densitiesof the components as well as the inverse Stokes number:

$\begin{matrix}{{{Density}\mspace{14mu} {Ratio}} \equiv \frac{\rho_{fluid}}{\rho_{particle}}} & (2) \\{{{Inverse}\mspace{14mu} {Stokes}\mspace{14mu} {number}} = \sqrt{\frac{2v_{f}}{\omega \; r^{2}}}} & (3)\end{matrix}$

Where ω is the frequency of vibration, v is the kinematic viscosity ofthe fluid, and r is the radius of the particle. It should be noted thatthe particle may have a lower density than the fluid, as in the case ofa bubble.

Decoupling that occurs between the components causes damping to occur inthe vibration of the tube, requiring more energy to maintain vibration,or reducing the amplitude of vibration, for a fixed amount energy input.

Turning to FIG. 3, a vapor pressure determination system 300 is providedaccording to an embodiment. A process line 303 having an inlet 304 andan outlet 307 is provided, wherein the process line 303 is configured tocarry a process fluid that enters the process line 303 through the inlet304. An upstream pressure regulator 308 is provided that controls thefluid flow through the process line 303. A downstream pressure regulator310 is provided that controls the fluid flow through the process line303. A flowmeter 5 having meter electronics 20 is disposed between theupstream pressure regulator 308 and the downstream pressure regulator310, and configured to receive process fluid that passes through theupstream pressure regulator 308. A pressure sensor 312 and a temperaturesensor 314 are also present in the system 300. Though the pressuresensor 312 and temperature sensor 314 are illustrated downstream of theflowmeter 5, these sensors 312, 314 may be situated before the flowmeter5, or incorporated within the flowmeter 5.

Meter electronics 20 is in communication with the upstream pressureregulator 308, downstream pressure regulator 310, pressure sensor 312,and temperature sensor 314. Meter electronics 20 may control theupstream pressure regulator 308 and downstream pressure regulator 310.Meter electronics 20 receives a pressure measurement from pressuresensor 312, and a temperature measurement from the temperature sensor314. The meter electronics 20 is configured to monitor the pressure ofthe process fluid, and reduce its pressure until the flowmeter 5 detectsthe introduction of a second phase, which indicates that the vaporpressure has been reached. In an embodiment, only a single pressureregulator 308 is present.

Turning to FIG. 4, a flow chart 400 is provided that illustrates anexample of a vapor pressure determination scheme employed by the system300. The pressure of the process fluid in the system 300 is measured instep 402. This is accomplished with the pressure sensor 312. Thetemperature of the process fluid in the system 300 is measured in step403. If the process fluid is single-phase under normal processconditions, the flowing pressure can be reduced by partially closing theupstream pressure regulator 308, as shown in step 404. Drive gain and/ordensity may be measured in step 406, and, as noted above, may beutilized to determine the presence of a multi-phase flow and also may beutilized to determine a monophasic/biphasic boundary. As the pressure ofthe process fluid is being measured 400, and the pressure of the processfluid is being reduced 404, the introduction of a second phase isdetermined via drive gain and/or density measurements 406, which in turnindicates that the vapor pressure has been reached. The detection of theflowing vapor pressure is indicated in step 408 by recording both thepressure and temperature at the point where the second phase isdetermined. In step 410, the RVP is calculated from the measured flowingvapor pressure taking into account the temperature at the time theflowing vapor pressure was recorded.

It should be noted that, if the process fluid already contains somevapor, this will be detected by measuring the drive gain and/or density,and the downstream pressure regulator 310 can be partially closed toincrease pressure for the purpose of determining the vapor pressure andtemperature at the point when the second phase is no longer present. Ineither case, it is the monophasic/biphasic boundary and the relatedtemperature/pressure of process fluid at this boundary that is utilizedto indicate the flowing vapor pressure of the process fluid.

In other embodiments, other pressure regulators and methods of pressurecontrol may be employed, should an upstream/downstream pressureregulator configuration not provide enough pressure change to reach thevapor pressure. In other embodiments, a temperature measurement couldalso be included, so to provide the ability to convert between truevapor pressure (TVP) and vapor pressure at standard temperature (e.g.Reid Vapor Pressure (RVP)). TVP is the actual vapor pressure of a liquidproduct at the measured temperature. TVP is difficult to directlymeasure and depends on the composition and temperature of the liquid inthe measurement device. Once the TVP and temperature are known, theflowing vapor pressure at any other temperature and/or the RVP can becalculated from the empirical correlation data stored in meterelectronics 20. The empirical correlation data may comprise look-uptables, mathematical algorithms, and/or mathematical curves. A directRVP measurement typically requires sending samples for laboratoryanalysis.

In an embodiment, the system 300 is disposed in a slip stream thatmeasures just a sample of the main flow stream, thus reducing impact onmaterial processes. Because RVP is largely dependent on composition, aslip stream sample will be effective in cases where composition isreasonably homogenous. This allows the system to be smaller in size,less costly, and less obtrusive.

The detailed descriptions of the above embodiments are not exhaustivedescriptions of all embodiments contemplated by the inventors to bewithin the scope of the invention. Indeed, persons skilled in the artwill recognize that certain elements of the above-described embodimentsmay variously be combined or eliminated to create further embodiments,and such further embodiments fall within the scope and teachings of theinvention. It will also be apparent to those of ordinary skill in theart that the above-described embodiments may be combined in whole or inpart to create additional embodiments within the scope and teachings ofthe invention.

Thus, although specific embodiments of, and examples for, the inventionare described herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The teachings providedherein can be applied to other vibrating systems, and not just to theembodiments described above and shown in the accompanying figures.Accordingly, the scope of the invention should be determined from thefollowing claims.

We claim:
 1. A method of determining vapor pressure of a fluid,comprising the steps of: providing a meter having meter electronics,wherein the meter comprises at least one of a flowmeter and adensitometer; flowing a process fluid through the meter; measuring apressure of the process fluid; adjusting the pressure of the processfluid until a monophasic/biphasic boundary is reached; and determiningthe flowing vapor pressure of the process fluid at themonophasic/biphasic boundary.
 2. The method of determining vaporpressure of a fluid of claim 1, wherein the step of adjusting thepressure of the process fluid until the monophasic/biphasic boundary isreached comprises lowering a pressure of a valve positioned upstream ofthe meter.
 3. The method of determining vapor pressure of a fluid ofclaim 1, wherein the step of adjusting the pressure of the process fluiduntil the monophasic/biphasic boundary is reached comprises raising apressure of a valve positioned downstream of the meter.
 4. The method ofdetermining vapor pressure of a fluid of claim 1, comprising the stepsof: measuring the temperature of the process fluid; and calculating theReid Vapor Pressure from the temperature and the flowing vapor pressure.5. The method of determining vapor pressure of a fluid of claim 4,wherein the step of calculating the Reid Vapor Pressure from thetemperature and the flowing vapor pressure comprises referencing ReidVapor Pressure values stored in the meter electronics using the ReidVapor Pressure from the temperature.
 6. The method of determining vaporpressure of a fluid of claim 5, wherein the Reid Vapor Pressure valuesstored in the meter electronics comprise a look-up table.
 7. The methodof determining vapor pressure of a fluid of claim 5, wherein the ReidVapor Pressure values stored in the meter electronics are calculatedfrom a curve.
 8. The method of determining vapor pressure of a fluid ofclaim 1, comprising determining the presence of entrained gas in theprocess fluid with a measured drive gain.
 9. The method of determiningvapor pressure of a fluid of claim 1, comprising determining thepresence of entrained gas in the process fluid by measuring a density ofthe fluid.
 10. The method of determining vapor pressure of a fluid ofclaim 1, comprising determining the presence of entrained gas in theprocess fluid with a combination of a measured drive gain and a measureddensity.
 11. A system (300) for determining flowing vapor pressure of aprocess fluid comprising: a meter (5) comprising at least one of aflowmeter and a densitometer; a pressure regulator (308) in fluidcommunication with the meter (5); a pressure sensor (312); meterelectronics (20) in communication with the meter (5) and the pressuresensor (312), wherein the meter electronics (20) is configured to:receive a measured pressure; control the pressure regulator (308) toadjust the pressure of the process fluid until a monophasic/biphasicboundary is reached; and determine the flowing vapor pressure of theprocess fluid at the monophasic/biphasic boundary.
 12. The system (300)of claim 11, wherein the meter (5) comprises: one or more conduits (103,103′); at least one driver (104) attached to the one or more conduits(103, 103′) configured to generate a vibratory signal to the one or moreconduits (103, 103′); and at least one pickoff (105, 105′) attached tothe one or more conduits (103, 103′) configured to receive a vibratorysignal from the one or more conduits (103, 103′).
 13. The system (300)of claim 11, further comprising: a temperature sensor (106) configuredto measure the temperature of the process fluid; and wherein the meterelectronics (20) is configured to calculate the Reid Vapor Pressure fromthe measured temperature of the process fluid and the flowing vaporpressure.
 14. The system (300) of claim 13, wherein the meterelectronics (20) comprises Reid Vapor Pressure reference values storedtherein.
 15. The system (300) of claim 14, wherein the Reid VaporPressure reference values stored in the meter electronics comprise alook-up table.
 16. The system (300) of claim 14, wherein the Reid VaporPressure reference values stored in the meter electronics are calculatedtherein.
 17. The system (300) of claim 11, wherein the meter electronics(20) is configured to determine the presence of entrained gas in theprocess fluid with a measured drive gain.
 18. The system (300) of claim11, wherein the meter electronics (20) is configured to determine thepresence of entrained gas in the process fluid with a measured density.19. The system (300) of claim 11, wherein the meter electronics (20) isconfigured to determine the presence of entrained gas in the processfluid with a combination of a measured drive gain and a measureddensity.